Being the first active circuit after the antenna in a radio transceiver, the low-noise amplifier (LNA) is a critical building block for a radio transceiver. To increase receiver sensitivity and reduce the amount of noise contributed by subsequent stages, the LNA is required to have a moderate gain and a low-noise figure. Typically, the LNA is preceded by a switch, duplexer, or filter and is very often followed by a filter. As a result, the LNA is required to be matched to the characteristic impedance of the system (usually 50 ohms) at its input and output.
When the input signal to the receiver is small, it is desired that the LNA provide maximum gain and minimum noise figure to increase receiver sensitivity. However, when the input signal to the receiver is large, the system can tolerate less gain and higher noise; thus, it is desired that the signal be routed around the LNA. Such an LNA is referred to as a switched-gain or bypassable LNA.
There are multiple examples of switched-gain LNAs known in the communications art. For example, U.S. Pat. Nos. 6,172,566, 6,175,274, 6,144,254, 6,522,195, 6,118,338 and 6,211,729, U.S. patent application publication No. US2002/0053947, and S. Reynolds et al., “A direct-conversion receiver IC for WCDMA mobile systems,” Proc. 2002 Bipolar/BiCMOS Circuits and Technology Meeting, pp. 61-64, September 2002, the disclosures of which are incorporated by reference, describe various LNA architectures.